Frost advisory issued for inland San Diego County

Throw an extra blanket on the bed. You'll need it tonight if you live in San Diego County's inland valleys.

The National Weather Service has issued a frost advisory that will be in effect from 1 a.m. to 8 a.m. on Tuesday. Temperatures are expected to drop into the upper 20s and low 30s, mostly in the mid-section of the county. Blame it on the cold, dry airmass that's moving through the region.

"Sensitive outdoor plants may be killed if left uncovered," says the weather service.

The U-T's Rob Krier reminds us that frost can occur even when the temperature is above 32 degrees. Says Krier: Some inland areas of San Diego County experienced frost Monday morning with temperatures in the mid 30s. Frost forms when a surface reaches 32 degrees. Some surfaces, depending on their composition, cool more than the air. A dark roof in the shade could have frost when the air temperature is 36 degrees. And in some cases, because cold air sinks, it can be a few degrees colder at ground level than five feet or more above, where most temperature readings are taken.

In San Diego County, frost is most likely west of the mountains on clear, wind-free nights and mornings. This type of frost is called radiation frost.

The weather may give you the shivers, but we're still experiencing some beautiful days -- something I was reminded of when I received this photo from Elizabeth Herman of Clairemont. She took the photo in Mission Trails.